In recent years it has become apparent that many invertebrate visual pigments have two stable states. Manipulation of the populations of these states by illumination at various wavelengths has made possible the dissection and analysis of the phototransduction process and furthermore has led to the discovery of new kinds of photoreceptor responses. A variety of spectrophotometric and electrophysiological techniques has been applied to these studies, which have been carried out on a number of different invertebrate preparations. The conditions of these preparations have also been manipulated in a number of ways, including the ionic environment, pH, the addition of various chemicals (alcohol, DNP, etc.), and the generation of photoreceptor mutants. This work has led to a wealth of phenomenology and little systematization. In this review we shall attempt to summarise and systematize the phenomenology, to present possible models for the underlying mechanisms, and to indicate directions of further research. In view of the need for such a systematic look at this field, the major journal Physiological Reviews has invited us to prepare a critical review, and the preparation of this review is the subject of the present proposal.